Differential PSK Signalling
Well I've managed to end up going down another tangent (was supposed to be finishing xmas shopping today!) and decided upon exploring how analogue modems work. I'm not really interested in developing my own Winmodem, just looking at the way the signalling works (slow signalling, so I can see the waves and stuff). ;)
What I'm looking at is developing a means of transferring stuff between two computers via their sound cards. Line In connected to Line Out and vice versa. I've settled upon trying to implement the ITU V.22 standard, which was created in the 80s and provided up to the princely baud rate of 1200. I'm looking to implement only the 600 baud mode!
Now, V.22 uses differential Phase-shift Keying (PSK). As they have to share the same phone line, the calling modem judders around on 1200 Hz and the answering modem on 2400 Hz.
I've had a go at creating a small program that takes any file as input and produces a wave file that certainly sounds "datary", but I've come across a couple snags:-
1. Requiring 80 samples per modulated bit produces massive wave files(!);
2. I've nothing to compare it against.
I can probably fix 1. if I knew what the lowest sample rate for this type of thing would still work, but 2. has me stumped. :confused:
Does anyone know where I can find example sound files of a V.22 transmission or other software capable of generating this stuff?